Network monitoring tools

Does anyone know of any good network monitoring tools? Need something that will run on Windows and will monitor and log bandwith, trafic coming inside/outside, what apps are using most of the bandwith . Our current network aplliances are all Cisco. We have Cisco 3750s switches, Cisco ASA 5510 and 5505 firewalls. Since last week our network seems to be suffering a lot e.g copying a file of about 1gb from one site to another. We got a 100mb point to point link. Problem seem to start from nowhere.

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